Injection molding typically involves the injection of molten resin through one or more gates to create multiple flow fronts within the mold. Flow fronts are the leading interface of a resin stream flowing within the mold. Multiple flow fronts may derive from multiple gates in the mold or from multiple flow paths communicating from a single gate. As the molding process progresses, the flow fronts eventually meet one another, resulting in the formation of one or more knit lines.
In the prior art, knit lines represent areas of decreased strength in the molded article. In molding processes that utilize fiber reinforcement in the resin, fiber materials tend to align in the direction of resin flow within the mold and typically do not become oriented or meshed across knit lines. As such, the use of injection molding to produce structural articles with fiber reinforced resins has been somewhat problematic. For example, reinforcement structures in vehicles are required to have rigid crossways linking the left and right sides together and sufficient strength to support bumper fascias, grille components, headlamps, radiator, washer fluid reservoir and other components of the vehicle. Conventionally, such reinforcement structures have been made from several stamped steel components welded together or from compression-molded continuous glass mat reinforced plastic. Although it has been recognized that injection molding techniques generally result in a less-expensive and lighter-weight end product than stamped-steel constructions or compression molding, conventional injection molding has not heretofore been widely employed for creating structural articles.
U.S. Pat. Nos. 5,556,650 and 5,417,916 teach an injection system using a single injection point for a fluid to displace a first portion of the molten plastic resin from a first portion of an article-defining cavity into a secondary portion of the article-defining cavity. There are limitations in using such a system having a single molten flow front to make complex shaped articles. For example, injection of a pressurized fluid may not adequately displace molten plastic resin through complex shaped articles. Further, knit lines can develop where flow fronts meet, resulting in decreased strength and structural integrity.
To summarize, articles that require increased strength and structural integrity, such as structural polymeric articles like reinforcing members for automobiles, are difficult if not impossible to make using conventional injection molding techniques.
Accordingly, it is a primary objective of the invention to provide molding processes and apparatus for making injection molded fiber-reinforced articles of increased strength as compared to articles made by prior art techniques.